1. Field of the Invention
The present application relates to nuclear magnetic resonance (NMR) imaging, and more particularly to a novel method of NMR imaging using bent slices in order to correct inherent slice distortion.
2. Description of Related Art
To perform NMR imaging it is necessary to produce gradients, or linear variations across space, in a static magnetic field, B.sub.0. This is achieved by pulsing current through coils designed to create such variations. If these applied gradients are not perfectly linear, distortions can result in an acquired NMR image. In NMR imaging, magnetic field gradients and radiofrequency energy are applied to a subject to be imaged causing a portion of the subject to resonate and enter an excited nuclear spin state. In many types of imaging, the excited portion has a planar or slice shape having a small thickness. The distortions induced by non-linear magnetic field gradients may occur both within the plane of the image (a readout direction and a phase-encoding direction), and in a direction through the plane or slice. Recent developments in gradient coil design have improved gradient linearity to some extent as described in U.S. Pat. No. 4,840,700 Current Streamline Method for Coil Construction issued to William A. Edelstein and John F. Schenck on Jun. 29, 1989 and assigned to the present assignee. Nevertheless, significant warping of the image can still occur. Novel gradient coils which trade linearity for speed or patient access can exacerbate this problem as described in U.S. Pat. No. 4,926,125 Surface Gradient Assembly for High Speed Nuclear Magnetic Resonance Imaging issued to Peter B. Roemer on May 15, 1990 and assigned to the present assignee. Distortions in the image in the readout and phase-encoding directions can be corrected by post-processing of the image, such as interpolating the image onto a non-distorted grid. Conventional techniques for correcting warped images after they have been acquired, correct only distortions within the image plane and cannot be used to straighten the slice itself, in the absence of contiguous slices. It is therefore desirable to provide a method for selecting an undistorted slice in the presence of nonlinear static-magnetic-field gradients in order to reconstruct an undistorted image.